A balun is a device designed to couple together balanced and unbalanced electrical signals. A balun can be considered a simple form of transmission line transformer. The most basic baluns use an actual transformer, with the unbalanced connection made to one winding, and the balanced to another. Other types of baluns use transmission lines of specific lengths, with no obvious transformer component. These are usually designed for narrow radio-frequency (RF) ranges where the lengths involved are some odd multiple of a quarter wavelength of the intended operating RF frequency. A common application of such a balun is in making a coaxial cable connection to a balanced antenna.
A balanced line or balanced signal pair is an RF transmission line that usually includes two conductors in the presence of a ground. The RF transmission line relies on balanced impedances to minimize interference. The RF signals on each line are typically the inverse of one another and each conductor is equally exposed to any external electromagnetic fields that may induce unwanted noise. The balanced line may be operated so that when the impedances of the two conductors at all transverse planes are equal in magnitude and opposite in polarity with respect to ground, the electrical currents in the two conductors are equal in magnitude and opposite in direction. These symetries can allow balanced lines to reduce the amount of noise per distance, which can enable longer cable runs. This is because electromagnetic interference will generally affect both signals the same way. Similarities between the two signals are automatically removed at the end of the transmission path when one signal is subtracted from the other. Balanced lines often also have electromagnetic shielding to reduce the amount of noise that may be introduced.
In contrast, an unbalanced line is a transmission line whose conductors have unequal impedances with respect to an electrical ground. Generally, in an unbalanced transmission line, one of the conductors is grounded.
Traditional narrow-band sleeve baluns generally use a quarter wavelength conductive cylinder. A coaxial (coax) cable is placed inside the conductive cylinder. At one end, the shielding braid of the coaxial cable is wired to the conductive cylinder while at the other end no connection is made between the cable and the conductive cylinder. The balanced end of the resulting balun is at the open end of the conductive cylinder, opposite from the end wired to the coax braid. At this point the coax cable separates into two conductors. One conductor is the center conductor separated from the braid, and the second conductor is the braid shielding of the cable or a connection to the braid. The quarter wavelength structure acts as a transformer converting the zero impedance at the end shorted to the braid to infinite impedance at the open end. This forces any current introduced by the balanced connection, such as a dipole antenna, to flow into the unbalanced coax connection as the infinite impedance of the cylinder prevents any currents from flowing on the outside of the coax cable. The conductive cylinder can be considered a choke structure. This type of balun is narrow-band or band-limited because the balun only functions well at odd multiples of quarter wavelengths. The baluns function particularly poorly at resonant frequencies (half wavelength multiples) where they may act as a short circuit.
In light of the bandwidth limitations of traditional narrow-band balun designs, there is a need for a balun system that operates over a very wide bandwidth and at millimeter RF frequencies. There is also a need in the art for a balun system that splits power splitter at the balanced end in order to support multiple balanced loads, such as multiple antenna elements. These wide bandwidth and power splitting qualities of a balun system are highly desirable in applications such as broadband, multiple-antenna communication systems.